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The Romulan Star Empire (or simply Romulan Empire) was a major interstellar state encompassing the Romulan people and their subject worlds and species. The Empire was known for its xenophobic character and policies of extreme secrecy, subterfuge, and territorial expansionism. By the late 23rd century, the Empire counted among the great powers in its region of the Milky Way Galaxy.

In external application, Romulan political agendas appeared largely unified over the centuries. Their goals focused on maintaining Imperial security through vigilance, and negating the perceived advantages of their rivals. Both goals were accompanied by a near-paranoid reticence to reveal information, even facts as basic as their racial identity, that might illuminate other Romulan ambitions or motivations. Short of war, the means to those ends have generally varied between favored methods:

After a clone intended to replace decorated Starfleet Captain Jean-Luc Picard, Shinzon, turned on the Romulans and committed a coup in 2379, the Federation (including Picard himself) helped stop Shinzon's plans of interstellar domination. The Romulans at that point suggested that an era of warmer relations may be beginning with the Federation. (Star Trek Nemesis)

The Romulan military was a powerful and technologically advanced force encompassing both spacefaring and ground forces. (TNG: "Unification II") Despite being defeated by Earth in the Earth-Romulan War of the 2150s, the Romulan Star Empire continued to be a powerful presence, not afraid to tempt Starfleet by entering the Neutral Zone. Romulan forces were a deciding factor in the Dominion War, whose entry in 2374 turned the tide of war in favor of the Allies. (TOS: "Balance of Terror"; DS9: "In the Pale Moonlight")

In dialogue, the Romulan Star Empire has been exclusively referred to as an Alpha Quadrant power. It has also been stated that the space of the Romulan Empire was surrounded by the territories of the Cardassians, Klingons and the Federation.

The first time this fact began to be established occurred in Star Trek: Deep Space Nine, in the episode "The Search, Part II", where the Romulan Star Empire was identified as one of the great powers of the Alpha Quadrant. For the Dominion, there were four great powers in this quadrant: the Federation, the Klingons, the Romulans, and the Cardassians. With the Battle of the Omarion Nebula, two of these threats had been reduced in effectiveness – the Romulans and the Cardassians. By "Call to Arms", the Cardassians were in an alliance with the Dominion and three powers, small and great, had signed non-aggression pacts with the Dominion. These powers were the Miradorns, the Tholians, and the Romulans. In the opinion of Odo, the Dominion was "making impressive inroads into the Alpha Quadrant."

In "In the Pale Moonlight", it was revealed that the Dominion forces were crossing the Cardassian-Romulan border and were launching attacks against Federation ships from there. When Benjamin Sisko was attempting to persuade Senator Vreenak to consider having the Romulans join the offensive against the Dominion, Sisko said, "I'd pick the side most likely to leave us in peace when the dust settles. Maybe you're right. Maybe the Dominion will win in the end. Then the Founders will control what we now call Cardassia, the Klingon Empire and the Federation. So, instead of facing three separate opponents with three separate agendas, you'll find yourselves facing the same opponent on every side. There's a word for that. Surrounded."

The association of Romulus with the Alpha Quadrant was carried over to Star Trek: Voyager. In "Flashback", CaptainKathryn Janeway reminisced about what it would be like to live in James T. Kirk's era, with "the Alpha Quadrant still largely unexplored," and "Romulans hiding behind every nebula." In "Message in a Bottle", a section of the Romulan border was identified to be located near the edges of the Alpha Quadrant. In "Bliss", when it seemed that the USS Voyager would find a way back home, Tom Paris stated that he looked forward to encountering Cardassians, Ferengi, and Romulans again in the Alpha Quadrant. In "Flesh and Blood", Chakotay identified the Romulan disruptor as an Alpha Quadrant weapon. In "Q2", in a simulation, several Alpha Quadrant species were fighting over mining rights to a planetoid, and Q Junior had to find a way to settle the dispute. The species listed were the Nausicaans, the Bolians, the Cardassians, the Romulans, the Ferengi, and the Bajorans.

In Star Trek Online, the Empire is in a state of inner turmoil after the destruction of their homeworld due to a supernova. They are now led by Empress Sela and blame the Federation for the destruction of their home planet, an event that led to a renewal of hostilities between the Romulan Empire and the Federation. Members of the Reman resistance and the Vulcan reunification movement have also established a Romulan Republic on "New Romulus" as a democratic state, allied with the Federation and the Klingon Empire in opposition to Sela's dictatorial regime. The new Romulan Republic play a major role in STO's first expansion, Legacy of Romulus.